1 John 3:1-3
3 See what great love the Father has lavished on us, that we should be called children of God! And that is what we are! The reason the world does not know us is that it did not know him. 2 Dear friends, now we are children of God, and what we will be has not yet been made known. But we know that when Christ appears,[a] we shall be like him, for we shall see him as he is. 3 All who have this hope in him purify themselves, just as he is pure.
For John the idea of being "born again" is a big one. The idea is that when someone believes in Jesus they are "born from above" and become part of God's holy family. Remember, in chapter 2, he calls the believers "little ones" or "children". John's gospel and his letters are very relational, which is why he focuses so much that the believers know God in a deep and intimate relationship. This is a great privilege for us, and those we have fellowship with in the body of Christ. Our love for and with God translates into our love with other brothers and sisters in Christ. But the world is still oblivious to this because they don't know God. They don't get it.
And we know in most families the children take on the family resemblance. Just the other day I saw a young man from my first congregation I served, that I hadn't seen in 20 years. Not only had he grown up, but he looked a lot like his dad. It was just a matter of time. Jesus, in effect, says the same thing today. As we grow in the family likeness and are conformed to the likeness of Christ, when he returns we will see Him just as He is. To the extent we have been formed in Him, we will be like Him.
If this is the goal to be like him and live like him, what are doing to grow to be more mature? In the second chapter it says as we obey him, we will know him and become more like him. And when he returns we will see him like He is. Remember the heresy John is confronting is those that say they can know Him but don't need to obey him. And that is a big problem in our churches today. We need to remember God has lavished his love on us so we can be called children of God. God has chosen us to be His children and called us to be like Him and the One He sent.